1. Field of the Invention
This invention relates to a method for measuring the concentration distribution of trace elements. More particularly, it relates to a method of analyzing and evaluating, with a high sensitivity and with high resolution, the existence and distribution of trace impurities contained in semiconductor materials, metallic materials, ceramics materials and organic materials in both depth-wise and planar directions.
The present invention also relates to an evaluation method for carriers in semiconductors and a method of preparing a standard sample. Speaking in further detail, the present invention relates to a method of evaluating the carrier concentration and the activation ratio of carriers in semiconductors, for example, by the measurement of the intensity of secondary ions, and a method of preparing standard samples used for this measurement method.
2. Description of the Related Art
Various processes have been attempted, by increasing the impurities in semiconductor materials, metallic materials, ceramic materials, organic materials, and so forth, or by deliberately adding impurity or impurities to them, to improve the material functional properties. However, analysis of trace elements contained in these materials is indispensable for evaluating these processes or evaluating the characteristic properties of the materials.
Secondary ion mass spectrometry (SIMS) analysis has been primarily used in the past as a method of evaluating trace elements existing in these materials, and ordinary methods comprise irradiating O.sub.2.sup.+ ions or Cs.sup.+ ions to the surface of a sample and detecting monoatomic ions (M.sup.+, M.sup.-) among the secondary ions emitted by sputtering. According to these methods, however, it has been extremely difficult to correctly and easily determine which elements exist in the film because the matrix effect, under which a detection sensitivity depends on the elementary composition of a base material, and the interface effect, under which the detection sensitivity changes at an interface of layers, are too powerfull. Although a method of detecting a two-atom composite ion (CsM.sup.+) between Cs.sup.+ and a detection object element (M) has also been proposed, the sensitivity by this method is not high, and this method cannot be applied to the analysis of trace elements. Accordingly, development of a method which can analyze trace elements in a solid has been required.
On the other hand, electronic devices such as Josephson junction devices and MOS ICS use an extremely thin oxide film as a tunnel oxide film or a gate oxide film. In these electronic devices, there exists a large number of problems with device fabrication relating to the oxide film such as the drop in performance such as withstand voltage or breakdown voltage of the oxide film resulting from heat-treatment and breakdown after ion injection, and information on element distribution in the oxide film is extremely important.
Secondary ion mass spectrometry has been employed mainly in the past as an evaluation method of elements existing in the oxide film in the depth-wise direction, and this method generally irradiates O.sub.2.sup.+ ions or Cs.sup.+ ions to the surface of a sample and detects the monoatomic ions (M.sup.+, M.sup.-) among the secondary ions emitted by sputtering. According to this method, however, it has been extremely difficult to accurately and easily determine information of the elements existing in the film because the matrix effect, under which the detection sensitivity depends on the elementary composition of the base material, and the interface effect, under which the detection sensitivity changes at the interface between the layers, are very strong. Recently, the method of detecting a two-atom composition ion of Cs.sup.+ and an object element has been proposed, but this method does not clarify the angle of incidence of Cs.sup.+ onto the sample surface. Accordingly, the reliability of the information thus acquired is not necessarily high.
On the other hand, when an element or elements existing on a fixed surface are to be determined, it has been customary to subject a sample to mass analysis without applying any pre-treatment (formation of an oxide film, etc.) to the sample. For this reason, the detection sensitivity is not sufficiently high, and there are many cases where trace elements, which were originally present, cannot be detected.
In a semiconductor device fabrication process, an electrically conductive impurity such as boron (B) of Group III or phosphorus (P) of Group V is doped into a semiconductor substrate by ion implantation or gas diffusion so as to form a P-type layer or an N-type layer and to obtain transistors, and so forth. To obtain the desired characteristics, control of a carrier concentration distribution is necessary. However, since the concentration distribution and the activation ratio of the conductive impurity change with the introduction method and with the heat-treatment condition, it is very important to correctly detect the carrier concentration distribution.
Spreading resistance analysis (SRA) is generally used at present as an evaluation method of the carrier concentration distribution in the semiconductors. This spreading resistance analysis is the method which brings two or four probes into contact with a sample which is subjected to oblique polish, to measure a resistance value, and converts the resistance value of the sample to the carrier concentration from the relationship of correspondence between the resistance value of a standard sample and a known carrier concentration. To obtain a concentration distribution in the depth-wise direction, further, the probes are sequentially moved on the polished surface in the depth-wise direction, and the resistance value corresponding to the depth is measured.
According to the spreading resistance analysis according to the prior art described above, however, the resistance value to be measured is affected by the crystalline plane orientation. Therefore, when the sample to be measured is polycrystalline, an error of several percent can occur in the measured resistance value. Further, resolution in the depth-wise direction is affected by the accuracy of the oblique polishing. Accordingly, the depth resolution is about 10 nm and an improvement in this resolution has been desired.
Further, secondary ion mass spectrometry (SIMS) is known as a method of measuring the quantity of conductive impurities. Generally, this method comprises bombarding oxygen ions, for example, as the primary ions to the sample to be measured, measuring the intensity of the secondary ions emitted from the sample, and specifying the quantity of the conductive impurity of the sample from the relationship between the intensity of the secondary ions of the standard sample having a known quantity of the conductive impurity and the quantity of the known conductive impurity. However, according to this method, an error occurs in the converted quantity of the conductive impurity if the standard sample is electrically charged and, eventually, an error occurs in the measured quantity of the conductive impurity. Though this method can measure the concentration distribution of the conductive impurity, it cannot measure the carrier concentration distribution.